Demonstrate an actual problem instead of a nebulous concern.
"Magic is a popular topic" is not a problem. This is like complaining that Stackoverflow has a lot of questions about C#, or that RPG.SE is dominated by D&D-related questions. MTG isn't crowding out the other games.
"B&CG makes a dedicated site seem less necessary" is not a problem. Or, rather, it's not a problem for B&CG. It may be a problem for that dedicated site. But that's part of the test a focused site has to pass in order to succeed in the first place. There are lots of tech-specific little spinoffs. Chess and Poker have beta SEs but we never went out of our way to ban those questions on B&CG.
Does B&CG adequately serve the MTG community?
Well, for starters, it serves the MTG community way more adequately than a B&CG site that tried to discourage Magic questions would. Our rules answers are timely and accurate. The volume of strategic discussion here is limited, but the strategy answers excellently relate a specific situation back to theory and fundamentals — which, in my honest opinion, is much better than what I've seen on many dedicated Magic sites.
And MTG is clearly a net benefit to B&CG as a whole — not just in terms of traffic and community but in how it drives the overall culture: low-speculation answers that rigorously cite rules whenever possible. You'll see MTG pop up a fair bit in the general game-design answers, as well, because it's one of those games whose individual elements contain a lot of different examples of mechanical interplay.